This invention relates in general to the aeration of waste water and more particularly to improvements in the mounting of diffusers on submerged air laterals in systems that treat municipal and industrial waste water.
A variety of diffusers have been used in waste water aeration, including fine bubble diffusers, flexible membrane diffusers, dome diffusers, porous tube diffusers and coarse bubble diffusers. The fine bubble diffusers are generally more efficient in transferring oxygen to the water, but they also have relatively high maintenance requirements. The coarse bubble diffusers are primarily applicable to low maintenance systems, while intermediate bubble systems represent a compromise between efficiency and maintenance requirements.
The aeration system typically includes submerged air laterals in the treatment basin arranged in the desired configuration. In the past, flexible membrane diffusers have typically been connected with the air laterals by a direct threaded connection between the diffuser and the lateral pipe. This requires outlets in the pipe which are internally threaded so that pipe nipples connected with the diffusers can be threaded into them. The major drawback with this diffuser mounting arrangement is that the air laterals must be constructed of heavy wall piping in order to provide enough threads to hold the diffuser in a cantilever position extending to the side of the lateral pipe. The need for heavy wall piping, whether stainless steel or polyvinyl chloride (PVC) adds significantly to the overall system cost.
In addition, the diffuser is subjected to turbulence, flexing, vibration and other forces while in service, and the stress applied to the diffuser connection is considerable. Ordinarily, the flexible membrane diffuser is about two feet long and the pipe nipple which connects to the air lateral is 3/4 inch in diameter. As a consequence of the fatigue that results from long term operation of the diffusers, the connections have a fairly high failure rate and the pipe nipples are actually sheared off in some cases. Diffusers more than about two feet long are not used because the stress increases with length and longer diffusers are unable to withstand the added stress.
Threaded connections are also subject to damage to the threads caused by screwing the diffusers in and out during maintenance procedures. Threads in plastic fittings can be cross threaded or otherwise damaged by the mating threads, especially if the male threads are metal. Because plastic threads are lacking in durability, the diffusers can work loose and fall out. Stress applied to the diffuser can lead to enlargement of the hole and other deformations which can create air leaks, and continued operation of the diffuser under these conditions can unscrew the diffuser and eventually result in its complete detachment from the aeration system.
Another problem with the direct threaded connection is that the diffuser is difficult if not impossible to accurately level. If the holes which are drilled and tapped in the sides of the lateral pipes are angled from a true radial orientation, the outboard end of the diffuser will be higher or lower than the inboard end. Also, if the tapped hole is rotated slightly on the pipe from a position exactly to the side, the diffuser will extend at a slight incline and the outboard end will again be too low or too high. Tolerances on the threads also cause out of level orientations of the diffusers. If the diffuser is not level, the air distribution pattern is disturbed because the outboard end either receives too little or too much air depending upon whether it is too low or too high. If diffusers extend to both sides of the pipe, one may extend down slightly and the other may extend up slightly so that an unbalanced situation results and the air distribution suffers accordingly. Units that screw into the top of the pipe are also difficult to level and have unbalanced air flow when out-of-level.
In conventional systems, it is difficult to add or relocate the diffusers because of the need for a threaded opening in the side of the pipe at each different diffuser location. The openings must be made at the factory and cannot be made adequately in the field. Moreover, when the threads are fully tightened, the diffuser is not necessarily located with its bottom side facing downwardly as required for proper diffusion of the air. Thus, if the diffuser is to be properly oriented, it must often be either over-tightened or under-tightened, neither of which is desirable. Overtightening can strip the threads or damage another part of the assembly, while under-tightening raises the possibility of the diffuser working itself loose and falling off of the air lateral due to vibrational forces or other forces applied to it in service. Units that require welded fittings are subject to similar problems.
Systems in which the diffusers connect directly to the sides of the lateral pipes necessarily locate the outlets on the horizontal center line of each pipe. When a large pipe four inches in diameter or more is used, the water is blown out only down to the level of the outlets. Consequently, separate water purge systems are needed to pump water out of the bottom half of the pipe in order for the aeration system to operate properly with minimum head loss. Such purge requirements add to the cost and complexity of the overall aeration system.
Tube type membrane diffusers are fully buoyant in that the entire diffuser is filled with air during normal operation. Although the fully buoyant system is easy and economical to produce, it also results in maximum stress being applied to the diffuser because the buoyant force on the diffuser is a function of the amount of water displacement which in turn depends upon the volume of the diffuser that is occupied by air. Therefore, in at least some applications, it is desirable to reduce the volume within the diffuser that is occupied by air in order to reduce the buoyancy stress to which the diffuser is subjected.
Coarse bubble diffusers are typically constructed of stainless steel, and they are often installed on stainless steel piping. Stainless steel diffusers and pipes are more costly than PVC and other plastics, and plastics are also less susceptible to corrosion problems. Again, direct threaded connections are sometimes used between the pipe and the pipe nipple of the diffuser, and this type of connection is lacking in structural strength. Adding or relocating diffuser units is difficult because the female outlet couplings must be factory welded to the stainless steel pipe. Leveling of the diffusers is also a problem caused by the manner in which they are connected to the air laterals.
In the past, various types of saddles have been proposed for effecting an outlet from an air header pipe. The known saddles that are constructed from PVC are solvent welded onto the top of the pipe with the saddles facing upwardly and having threaded outlets. Special flat plate diffusers are screwed directly into these outlets. Due to the solvent weld required to connect the saddle to the pipe, this type of saddle can be used only with PVC pipe and not with stainless steel or many other materials. Thus, when a particular application calls for stainless steel pipe, the saddles cannot be used. It is common for stainless steel straps to be used to secure the saddle, even when a glue connection is provided.
Conventional coarse bubble diffuser systems require an orifice between the air lateral and the diffuser in order to provide a pressure differential that prevents downstream diffusers from being deprived of significant air flow. The orifice is normally located in the inlet to the diffuser where it is subject to becoming clogged when the air is discontinued and waste water backs up into the diffuser. Solids that flow back through the orifice can become trapped and considerable amounts of debris can accumulate and cause flow disruptions.